The invention relates generally to mass spectrometers, and more specifically to apparatus and methods utilizing ionic liquids for air-to-vacuum sealing of mass spectrometers and for ion transport of ambient ions into a mass spectrometer.
The use of mass spectrometers in the field for analyzing environmental samples at ambient pressures is limited by the lack of truly portable mass spectrometers. Portability, particularly for military use, means battery operation for a soldier already carrying close to one hundred pounds of equipment. The primary hurdle is the need for heavy vacuum pumps and multiple vacuum stages in a typical mass spectrometer.
The sensitivity and specificity of mass spectrometers for detecting trace species is unparalleled. In mass spectrometers, samples are ionized and their ion trajectories in an electric and/or a magnetic field measured to determine the mass-to-charge ratio of the ions. The observed mass spectra allow identification of the original samples.
The development of Electrospray Ionization (ESI) allowed highly non-volatile samples such as biomolecules to be analyzed by mass spectrometers, greatly extending their utility. The development of ESI for analysis of biological macromolecules was rewarded with a Noble Prize in Chemistry to John Bennett Fenn in 2002.
The use of mass spectrometry to study environmental samples in the field, however, is limited both by equipment size and weight and by sample preparation times.
The problem of sample preparation times has been reduced by the development of a highly sensitive mass spectrometric approach for probing surface adsorbates at ambient pressures without a need for sample preparation. The method, described in U.S. Pat. No. 7,335,897 to Takats et al. called Desorption Electrospray Ionization (DESI), exposes a surface of interest to an ESI source that produces ions and charged droplets that, when impinging on the surface, desorb surface material that ionize in the spray. The desorbed ions are then collected by the inlet system of a conventional mass spectrometer for analysis. DESI, as well as ESI, are known as soft-ionization approaches in which the resulting ions experience little fragmentation, thereby minimizing spectral congestion. DESI mass spectra resemble closely ESI mass spectra, where the sample is dissolved in a volatile solvent that is electrosprayed into a mass spectrometer.
That still leaves the problem of the size and weight of mass spectrometers as a limiting factor for their use to study environmental samples in the field.
By whatever means an ambient sample is ionized, it must then be introduced into the inlet of the first vacuum stage of a traditional mass spectrometer. Because the inlet is exposed to the atmosphere, maintaining a sufficient vacuum throughout the process path of a typical mass spectrometer requires as many as five differently sized pumps to maintain a vacuum.
The process path of a typical mass spectrometer may include three differentially pumped chambers, a first pumping stage usually involving a powerful mechanical pump and two high vacuum stages, the first for ion transport and possibly cooling, and the second housing the mass analyzer and detector. The second stage may be omitted if the mass analyzer does not require highly thermalized ions.
The vacuum system is the largest, heaviest and most power-consuming component of a mass spectrometer and represents the main impediment to making a portable ambient mass spectrometer.
If the flow of atmospheric gases into a mass spectrometer could be eliminated, only a turbopump and a low power roughing pump, such as a diaphragm pump, at most, would be needed. These two pumps could even be smaller than those in a differentially pumped system due to the negligible load, outgassing from internal components being the main source of gasses. A well outgassed system, that is, a system connected to a vacuum pump sufficiently long for outgassing to complete, could be operated without pumps for a significant length of time.
It is, therefore, an object of the invention to provide an atmospheric seal for the inlet of a mass spectrometer.
Achieving that object, however, still leaves a problem of transporting ions from the DESI technique into the mass analyzer section of a mass spectrometer.
It is, therefore, another object of the invention to provide an atmospheric seal for the inlet of a mass spectrometer that permits passage of sample ions through the seal into the mass spectrometer.
In a separate area of research, ionic liquids, or room temperature molten salts, are liquids consisting entirely of ions and have unique properties making them attractive for a large number of industrial applications. Among those attractive properties are their negligible vapor pressures and their versatility as solvents for both inorganic and organic materials. Those properties enable electrospraying ionic liquids in a vacuum, contrary to the types solutions used in a typical ESI experiment. In a vacuum, the volatility of conventional ESI solvent solutions, where volatile solvents are used to carry solutes, would compromise both the vacuum as well as causing freezing of the solution, thereby discontinuing the flow of the spray.
A team of researchers, including one of the inventors of this invention, recently discovered that certain ionic liquids can be electrosprayed in a pure ion emission mode, that is, the spray does not include charged droplets, but only ions. The discovery is described in “Mass Spectrometric Analysis of Colloid Thruster Ion Emission from Selected Propellants,” Y. Chiu, B. L. Austin, R. A. Dressler, D. Levandier, P. T. Murray, P. Lozana and M. Martinez-Sanchez, J. Prop. Power, 2005, 21, 416-423, which is incorporated by reference into this application.
In an additional development, two other members of that team have discovered that ionic liquids can be sprayed into a vacuum from wetted sharp needle tips. This external wetting approach, coupled with small and sharp needles, forces many ionic liquids to emit in a pure ion emission mode. Further, extraction voltages can be as low as 1 kV, significantly enhancing the miniaturization potential of an ESI source. This discovery is described in “Ionic Liquid Ion Sources: Characterization of Externally Wetted Emitters,” P. Lozano and M. Martinez-Sanchez, J. Coll. And Interface Sci., 2005, 282, 415-421, which is incorporated by reference into this application.